The following discussion of the state of the art should not be construed as an admission of prior art.
Duplex steel is characterized by a ferritic-austenitic structure, where both phases have different compositions. Modern duplex steel will mainly be alloyed with Cr, Mo, Ni and N. The duplex steel grade SAF 2507 (UNS S32750) has high contents of Cr, Mo and N for a high resistance to pitting corrosion. This resistance is often described as a PRE-number (PRE =Pitting Resistance Equivalent=%Cr+3.3% Mo+16N). The alloy is consequently optimized with respect to this property and has a good resistance in many acids and bases, but the alloy is above all developed for resistance in chloride environments. The elements Cu and W have been used as alloying additions. Consequently has for example the steel grade DP3W (UNS S39274) which has an analogous composition as SAF 2507, but it is alloyed with 2.0% W as substitute for a share of the Mo-content in the alloy. The steel grade Uranus 52N+(NS S32529) has similarly an analogous composition as SAF 2507, but it is alloyed with 1.5% Cu with the purpose to improve the resistance in acid environments. The steel grade Zeron 100 is a further steel grade which is analogous to SAF 2507, but this is alloyed with both about 0.7% Cu and 0.7% W. The steel grade DTS 25.7NWCu (UNS S39277) is in this composition very similar to SAF 2507, besides that it is alloyed with about 1.7% Cu and 1.0% W. In relation with that it is alloyed with W, a PRE formula was produced, which also includes the element W with a weight corresponding the halve of this for Mo., i.e.--PRENW=%Cr+3.3(%Mo+0.5% W)+16N. All described steel grades have a PRE number, irrespective to the calculation method, that is over 40.
Another type of ferritic-austenitic alloy with high resistance to chloride is the steel grade described in the Swedish Patent 9302139-2 or U.S. Pat. No. 5,582,656. This type of alloy is characterized by Mn 0.3-4%, Cr 28-35%, Ni 3-10%, Mo 1-3%, Cu maximum 1.0% and W maximum 2.0%, and has a high PRE number, generally over 40. The biggest difference compared with the established superduplex steel SAF 2507 and others is that the contents of Cr and N are higher in this steel grade. This steel grade has been used in environments where the resistance to intergranular corrosion and corrosion in ammonium carbamates is of importance, but the alloy also has a very high resistance to chloride environments.